Metallic thin films are used extensively in a variety of electronic devices and applications. They are used to make electrical conducting paths to interconnect various parts of an electrical device. Such conducting paths might also be used to create stationary or moving magnetic fields and/or electrostatic fields. Such metallic thin films might also be used as an electrical contact surface or as a bonding surface (bonding pad) in various types of devices. Metallic thin films are used in various memory devices such as magnetic bubble devices as well as in a variety of integrated circuits. A variety of metals and alloys are used for metallic thin films including aluminum, gold, titanium-gold, molybdenum-gold, titanium-platinum-gold, etc. By far, the most extensively used material is aluminum.
The recent trend toward increasing component density and additional miniaturization in integrated circuits and memory devices has greatly increased current densities in conducting paths. At such high current densities, electromigration becomes a serious reliability problem. Ames et al. (U.S. Pat. No. 3,725,309) suggested a procedure for greatly reducing the electromigration of aluminum. They observed that the introduction of small amounts (typically 0.1 to 10 weight percent, usually 4 to 6 weight percent) of copper into the aluminum greatly decreased the electromigration of aluminum and increased the lifetimes of conductive stripes in integrated circuits and other electrical devices.
In electronic circuits and memory circuits, thin films are usually patterned by covering the area with a metal layer and removing unwanted metal often by chemical etching. For aluminum-copper metal layers, acid etches are most often used. Typical acid etches are phosphoric acid generally mixed with nitric acid and acetic acid.
Such etches yield excellent results for the majority of applications. However, in some applications, an acid etch cannot be used. Usually, this situation arises where some necessary part of the circuit is attacked by acid etches. A particular case in point is in the fabrication of magnetic memory circuits such as bubble memories. In these devices, it is often necessary to incorporate a layer of magnetic material (i.e., permalloy) in the device. Acid etches cannot be used because such etches often attack magnetic material.
In the case of pure aluminum, various alkaline etches such as aqueous NaOH or aqueous KOH give excellent results. In the case of aluminum-copper alloys, some residue remains which is usually removed by agitation or abrasion. It is highly desirable to have an etching process where the entire alloy is dissolved in the etching solution. Such a process would avoid exposing the various devices to abrasive manufacturing operations and remove the possibility that the residue would interfere with operation of the device.